1. Field of the Invention
The present invention relates to a production method of a resist composition for lithography, and specifically to a production method of a resist composition for lithography capable of decreasing occurrences of defects such as coating defects and pattern defects.
2. Description of the Related Art
With enhanced integration degrees and increased speeds of LSI's, miniaturization of pattern sizes is being rapidly progressed. The lithography technique has achieved formation of finer patterns, by appropriately adopting a light source having a shorter wavelength and a resist composition suitable therefor, in step with such a scaling down. Along this trend, resins to be used for photoresist compositions have been required to be low in light absorption at exposure wavelengths, respectively, resulting in a transition of resins from a novolak resin through polyhydroxystyrene to a resin having an aliphatic polycyclic structure along with a progressive transition of exposure light from i-line through KrF to ArF.
Further, in case of scaling down using the same light source in a manner to achieve the scaling down while keeping a thickness of an adopted photoresist film as it is, i.e., in a manner to merely narrow pattern line widths, the photoresist pattern after development is increased in aspect ratio of line profile, thereby resultingly causing a pattern collapse. Thus, along with the scaling down, photoresist films have been decreased in thickness such that aspect ratios of line profiles of photoresist patterns are kept in appropriate ranges, respectively.
Meanwhile, for processing a substrate to be processed, such a method is typically adopted to use a patterned photoresist film as an etching mask, in a manner to process the substrate to be processed by dry etching. This means that substrates to be processed are required to be dry etched by using photoresist films each made to be thinner and thus weaker in etching resistance due to the above circumstances, thereby requiring to promptly ensure obtainment of a material and the like to be used in such a processing step.
Examples of methods for solving such a problem include a multi-layer resist method. This method is configured to: place a resist lower layer film between a photoresist film, i.e., resist upper layer film, and a substrate to be processed, the resist lower layer film having an etching selectivity different from that of the resist upper layer film; obtain a pattern in the resist upper layer film; subsequently transfer the pattern into the lower layer film by dry etching, by using the upper resist pattern as a dry etching mask; and transfer the pattern into the substrate to be processed by dry etching, by using the lower layer film as a dry etching mask.
In this way, resist compositions to be used in a lithography process have been decreased in film thickness and increased in the number of layers, in forming fine patterns. Then, coated films to be formed from compositions to be used at this time are decreased in film thickness, so that even minute defects tend to affect a yield of a process.
To decrease such defects in a composition for liquid immersion exposure, there has been exemplarily known a method for decreasing an amount of dissolved gas in the composition (see Japanese Patent Laid-Open (kokai) No. 2010-020297).